Synthesis and Processing of Organic Materials in Supercritical Carbon Dioxide
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Processing of Organic Materials in Supercritical Carbon Dioxide
Georgia E. McCluskey, Jin-Kyun Lee, Jing Sha, Christopher K. Ober, Scott E. Watkins, and Andrew B. Holmes Abstract Carbon dioxide (CO2) is arguably the most high-profile molecule of recent times. Although much of its bad press comes from environmental concerns associated with greenhouse gas emissions, there exists the possibility to harness this abundant resource for application to the synthesis and processing of useful materials. This article describes a selection of recent successes in using supercritical carbon dioxide (scCO2) as a solvent for polymerizations. Further, the uses of compressed CO2 as a processing tool in the fabrication of materials for applications such as coatings and biomaterials are discussed. Finally, the application of scCO2 to photolithography is demonstrated, with particular focus on CO2 as a processing solvent for the patterning of organic electronic devices.
Introduction An increasing sense of responsibility for the environment has led to concerns about large-scale chemical processes that result in gross waste production.1 Much of this waste consists of volatile solvents used in the manufacture and purification of bulk and fine chemicals. In addition to the environmental hazards, health and safety are also of concern, given the highly flammable nature of many volatile solvents.2 Research into alternative solvents for chemical processing has uncovered CO2 (liquid or supercritical) as an alternative to organic solvents.3 Unlike common organic solvents, CO2 is nonflammable and much less toxic than solvents such as acetone and chloroform.2 It is also relatively chemically inert, meaning that it is not likely to react with compounds when it is used as the reaction medium or processing solvent. CO2 is readily available, as it is a common byproduct of many industrial processes such as the production of 108
ammonia and ethanol and the refinement of fuels.2 Although CO2 is clearly a greenhouse gas, the concept of making use of industrial sources might be considered a method of sequestering it.2 In addition, there is potential for recovery and recycling in a closed-loop process.4 Supercritical CO2 (scCO2) has attracted much interest given the mild conditions required to reach the supercritical state. This is achieved when CO2 is heated above its critical temperature (Tc = 31.1°C) and compressed above its critical pressure (Pc = 7.38 MPa)5 (Figure 1). Supercritical CO2 has unique physical properties, including liquid-like densities, allowing it to be used as a solvent for reactions and chemical processes requiring solvation of compounds. Simultaneously, it demonstrates gas-like diffusivity and viscosity, making it potentially ideal for processes such as small-dimension photoresist development6 and dry cleaning, as the
low surface tension means that no damage to materials occurs and, unlike for conventional methods that create high volumes of waste, there is no net environmental impact.7 This article reviews the use of CO2 as a solv
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